Diffusing wave spectroscopy (DWS) is the optical technique which studies the dynamics of a scattering medium by measuring multiply scattered light. The term of diffuse correlation spectroscopy (DCS) is used instead of DWS. Both work in the multiple scattering regimes. However, the term DCS has been extended to heterogeneous multiple-scattering media. For simplicity, the name of DWS is used herein and includes both techniques.
In general operation for a DWS system, a laser light source 1 outputs the laser light beam with long coherence length. For example, the coherence length of the laser light beam is about 10 m. The laser light source is coupled to a multimode fiber (MMF) 2. The light is irradiated to the sample 3 via the MMF 2. The irradiated light is multiply scattered in the sample 3. A part of the multiply scattered light is detected by a single mode fiber (SMF) or a few mode fiber 4. The distance between the MMF 2 and the SMF 4 depends on the depth of interest. The detected light is input to a photon-counting avalanche photodiode (APD) 5. The intensity autocorrelation function is calculated using the correlator 6 and processed in a computer 7. The DWS may have plural detection system. For example, see David A. Boas, et al., Handbook of Biomedical Optics, CRC Press, Chapter 10, 2011.
The decay time which is calculated from the intensity autocorrelation function is related to the viscosity of the sample and/or the flow of particles in the sample. Therefore, for example, DWS has been used for estimating blood flow in the tissue. See G. Dietsche, M. et al., Appl. Opt., Vol. 46, No. 35, 8506-8514, 2007.
However, the performance of DWS systems are significantly degraded by noise. One significant type of noise is dark current. The dark current is the electric current output from a detector without light input to the detector. Other sources of noise include light which comes from other light sources. Room light and the sunlight are typical noise light. The light emitted from another laser light source is also noise light and can be a significant source of noise in some applications of DWS. To reduce noise light, the measurement may be performed in a darkroom or when the detection system is shielded carefully. Also, the narrow optical bandpass filter can be used for decreasing the noise light. However, each of these solutions have their own drawbacks—such as requiring a darkroom in a doctor/patient setting.
In order to avoid the influence of dark current to DWS measurement, the photon-counting photomultiplier tube (PMT) or avalanche photo diode (APD) with low dark current (dark count rate) are often employed. In Dietsche et al., an APD with the maximum dark count rate of 500 cps (SPCM-AQ4C, PerkinElmer) was used. This system employed separate detection of plural speckle grains, plural intensity autocorrelation function curves that were calculated in parallel and averaged. By using this technique, the measurement time can be reduced. However, plural photon counting APDs and plural correlators were needed, adding significant cost and complexity to the system.
Thus, there is need for in DWS to provide a low noise system at a low cost.